The present invention relates in general to a variable electronic component, and more particularly, to a variable trimmer capacitor adapted to provide a high resolution operative range of impedance characteristics, i.e., capacitance (pF), at greatly increased voltage ratings while eliminating possible failure modes.
In the advancing technologies of computers, testing equipment, appliances and other fields, various types of hybrid electronic circuits, integrated electronic circuits, micro strip amplifiers, micro electronic systems and other electronic devices are being developed and which are being made on increasingly smaller scale and size then has been known heretofore. In accomplishing this end, these devices are generally mounted on small flat insulating bases known as substrates. These various devices incorporate or have formed therein or co-act with various types of electronic components such as inductors, capacitors, resistors, potentiometers, etc., which in turn must be as small as possible to meet the demands of the reduced scale or size of these devices.
The known miniature devices have reached their functional limit even with the best precision manufacturing techniques because certain elements of these devices cannot be reduced further at reasonable cost for commercially acceptable regularly usable devices. U.S. Pat. Nos. 3,679,940, 3,757,266, 4,575,779, 4,764,843 and 4,876,627, which patents are assigned to the same assignee of the within invention, each disclose variable electronic components, such as trimmer capacitors and resistors, which are particularly adapted to meet and overcome the problems of these known devices by eliminating the area and mass of conventional mounting frames, tuning mechanisms and other elements heretofore used, and instead integrate the elements of these variable electronic components directly into the circuit. These components are primarily designed as low-profile structures exceedingly small in size, simple in construction, easily adjustable, reliable in operation, easily replaceable and reproducible in quantity without sacrificing uniformity or performance.
In Blickstein, et al., U.S. Pat. No. 3,483,450, assigned to the same assignee of the within invention, there is known a concentric ring air trimmer
capacitor useful in high frequency circuits because of its high Q, its low self inductance and its self-resonant frequency which is well above one GHz. The capacitor of Blickstein, et al., shown in FIG. 1 includes a cylindrical housing 100 of insulating material, such as ceramic, which is metallized at its ends. Positioned within the housing 100 is a stator capacitor plate or stationary electrode 102 which is formed of concentric cylindrical sleeves 104 extending from an end plate 106. The entire unit may be precision die-cast or machined of metal, as for example, zinc. The stationary electrode 102 through the end plate 106 is in electrical contact with the metallized ends of the housing 100. The center of the stationary electrode 102 is in the form of a slide post 118 on which is positioned a plastic guide bushing 110. A metal bushing 112 which may also be precision die-cast or machined is secured to the other end of the housing 100. The bushing 112 forms an extension of the housing 100. The interior of the bushing 112 is provided with longitudinally extending guide slots (not shown). All but two opposed of these guide slots do not extend entirely through the bushing and are provided with blind end shoulders.
An adjustable impedance varying electrode 114 is provided for sliding actuation within the bushing 112. The impedance varying electrode 114 is in the form of a number of concentric cylindrical sleeves 116 Which can nest concentrically within the sleeves 104 forming the stationary electrode 102 with an air gap 115 therebetween to vary the impedance characteristics of the capacitor. The impedance varying electrode 114 may also be precision die-cast or machined of metal, axially drilled and tapped with threads at 118. The end of the impedance varying electrode 114 is furthermore provided with guide extensions or lands (not shown), which engage in the guide slots of the bushing 112 to allow sliding of the impedance varying electrode without rotation. A spring washer 120 is provided with spring contact arms 122 extend in the guide slots which extend entirely through the bushing 112. The end of the bushing 112 is provided with an annular cut-out which forms shoulders between the slots. Movement of the spring washer 120 toward the stationary electrode 102 is prevented by the spring washer being seated against these shoulders. Spring washer 120 is preferably of beryllium copper and the contact arms 122 are preferably bent as shown so as to ensure sliding contact with the impedance varying electrode 114 and also to be maintained in electrical contact with the inner wall of the bushing 112.
A drive screw 124 extends coaxially into the bushing 112. The end of the drive screw 124 is provided with an enlarged head or flange 126 which engages the washer 120 in order to prevent axial movement of the drive screw toward the housing end. The outer end of the flange 126 engages a washer 128 which is held in place by the turned in edge or lip of the end of the bushing 112. A sealing ring 130 surrounds the flange 126 in order to provide sealing between the flange and the bushing 112. The drive screw 124 is provided with a conventional slotted head for rotation with a screw drive. The drive screw 124 is thus rotatably captured in the bushing 112 and with the sealing ring 130 forms a rotating seal, sealing the end of the bushing 112 and the entire interior of the unit.
In the Blickstein, et al. capacitor, the capacitance is determined by the dimensions of the air gap 115 between the stationary and impedance varying electrodes 102, 114. Because this air gap is on the order of 0.003 inches and must be maintained uniformly throughout the travel of the impedance varying electrode, the tolerance and a manufacturing process must be tightly controlled. As these tolerances have a finite limit, it is not possible to extend the capacitance range beyond about 16 pF as the resulting dimension of the air gap 115 would be inadequate to prevent shorting between the stationary and impedance varying electrode 102, 114. This problem is specifically severe in high RF voltage applications where arcing in the air space may be more prevalent.
In an apparent effort to address these problems, Blickstein, et al. has contemplated coating the stationary and impedance varying electrodes 102, 114 with thin layer of plastic dielectric material, such as Paralene. However, this modification still requires the presence of the air gap 115 to constitute the dielectric medium for the capacitor.
In Davidian, U.S. Pat. No. 4,953,057, there is disclosed a capacitor constructed from a pair of opposing spiral coils made from a continuous flexible
thin film of dielectric plastic material such as Mylar.RTM., polycarbonate or polypropylene which has been coated on one side with a thin layer of conductive material such as aluminum. The Davidian capacitor, based upon thin film technology, suffers from a number of notable drawbacks. For example, the capacitor is inherently unstable with respect to vibration and shock, as well as being inoperative at higher frequencies. In addition, as it is a requirement that the spiral coils be at least always partially nested, this results in a higher minimum capacitance which may be undesirable in certain applications. Still further, the use of continuous spirals having an extensive number of turns renders the capacitor complicated to manufacture and assemble.
Accordingly, it can be appreciated that there is an unsolved need for a variable electronic component, such as a variable trimmer capacitor, which can provide a desired high resolution operative range of impedance characteristics quickly, simply and inexpensively at substantially increased voltage ranges.